Heimdall

A cross-platform process sandbox runtime written in Rust. Heimdall executes untrusted commands with configurable filesystem, network, and environment isolation — using bubblewrap and Linux namespaces on Linux, and Seatbelt on macOS.

Features

• Cross-platform isolation — bubblewrap on Linux, Seatbelt on macOS
• Filesystem isolation — deny patterns, writable overlays, and virtual file injection using gitignore-style globs
• Network isolation — disable host networking inside the sandbox
• Environment filtering — allowlist or blocklist parent environment variables
• Process hardening — disable ptrace attachment, zero core file limits
• /proc control — mount or hide /proc inside the sandbox (Linux)
• Fragment files — .heimdall-deny and .heimdall-write files in your project root for local policy overrides
• Protected targets — .git, .agents, .pi, and .heimdall-* paths are automatically write-protected
• JSON policy documents — declarative sandbox configuration with schema validation
• stdio control — inherit or pipe child process I/O

Installation

Homebrew (macOS and Linux):

brew install casualjim/homebrew-taps/heimdall-sandbox

Shell installer:

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/casualjim/heimdall-sandbox/releases/latest/download/heimdall-sandbox-installer.sh | sh

npm (macOS Apple Silicon, Linux x64, and Linux arm64):

npm install -g @casualjim/heimdall-sandbox

Linux arm64 remains a supported sandbox/package target, but Linux arm64 releases are built without WebGPU. The privacy-filter WebGPU execution provider currently depends on upstream ONNX Runtime/Dawn prebuilt artifacts that are not available for Linux arm64, so Linux arm64 uses the CPU execution provider unless a source-built or vendored Linux arm64 WebGPU runtime is added.

Cargo registry:

cargo install heimdall-sandbox

Quick start

Run a command inside the sandbox:

heimdall-sandbox exec -- printf "hello"

You should see hello printed to your terminal. The sandbox inherited your current directory, environment, and stdio — nothing was isolated yet.

Now restrict the environment so only PATH reaches the child:

heimdall-sandbox exec --allow-env PATH -- env

Notice that most variables are gone. This is allowlist mode: only explicitly named keys pass through.

How to ...

Restrict environment variables

Allowlist mode (only pass listed vars):

heimdall-sandbox exec --allow-env PATH --allow-env HOME -- env

Blocklist mode (pass everything except listed vars):

heimdall-sandbox exec --deny-env SECRET_TOKEN -- env

--allow-env and --deny-env are mutually exclusive.

Disable networking

Pass a JSON policy document via stdin:

heimdall-sandbox exec --policy - <<'EOF'
{
  "network": "none",
  "command": ["curl", "https://example.com"]
}
EOF

The child will fail because the network is unreachable inside the sandbox.

Hide files and inject virtual ones

Filesystem policies use gitignore-style glob patterns. This example hides .env files, marks src/ as writable, and replaces /etc/passwd with a virtual file:

heimdall-sandbox exec --policy - <<'EOF'
{
  "command": ["cat", "/etc/passwd"],
  "filesystem": {
    "deny": ["**/.env*", "!**/.env.example"],
    "writable": ["src/**"],
    "virtual": {
      "/etc/passwd": "nobody:x:65534:65534:Nobody:/nonexistent:/usr/sbin/nologin\n"
    }
  }
}
EOF

Override policies locally with fragment files

Place a .heimdall-deny or .heimdall-write file in your working directory to extend the policy without changing the JSON document. Patterns in these files are appended after the JSON patterns, so negations can selectively re-allow denied paths:

# .heimdall-deny — deny all dotfiles except .env.example
.env*
!.env.example

Fragment files are discovered automatically — no CLI flag needed.

Validate or inspect a policy document

Validate a policy file before running it:

heimdall-sandbox policy validate policy.json

Print the JSON schema to author policies with editor support:

heimdall-sandbox policy schema

Policy document reference

How it works

Heimdall is a Cargo workspace with four crates:

Requirements

• Linux (x86_64, aarch64) — filesystem and network isolation require bubblewrap and user namespaces
• macOS (Apple Silicon) — filesystem and network isolation use the built-in Seatbelt sandbox
• Rust 2024 edition (1.85+) for building from source

License

MIT